1. Field of the Invention
The present invention relates to an air-fuel ratio control device and an air-fuel ratio control method for an internal combustion engine.
2. Description of the Related Art
Exhaust gas discharged from an internal combustion engine contains components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). A three-way catalyst is used to convert these components to less toxic substances. The performance of such a three-way catalyst increases when the air-fuel ratio of the exhaust gas (hereinafter, referred to as “exhaust air-fuel ratio”) is substantially stoichiometric. Thus, to purify exhaust gas using a three-way catalyst, the amount of fuel supplied to the combustion chamber is controlled so that the exhaust air-fuel ratio is substantially stoichiometric.
To this end, in most internal combustion engines, an air-fuel ratio sensor that detects the exhaust air-fuel ratio is provided in an engine exhaust passage upstream from the three-way catalyst. Feedback (F/B) control is performed to control the amount of fuel supplied to the combustion chamber so that the exhaust air-fuel ratio detected by the air-fuel ratio sensor is substantially theoretical.
However, on the upstream side of the three-way catalyst, the output of the air-fuel ratio sensor may become unstable due to insufficient mixing of exhaust gas, or the air-fuel ratio sensor may degrade due to the heat of exhaust gas, making it impossible for the air-fuel ratio sensor to accurately detect the actual air-fuel ratio. In these cases, the accuracy of air-fuel ratio control based on the above-described feedback control deteriorates.
In view this, a so-called “double sensor system” has already been put into practical use. In the double sensor system, a second air-fuel ratio sensor is provided in the engine exhaust passage downstream from the three-way catalyst. The double sensor system improves the accuracy of air-fuel ratio control by performing a sub-feedback control, which corrects the output value of the upstream air-fuel ratio sensor (and consequently the amount of fuel supplied) based on the output of the downstream air-fuel ratio sensor so that the output value of the upstream air-fuel ratio sensor matches the actual exhaust air-fuel ratio.
In this double sensor system, a learned value corresponding to a steady-state error between the output value of the upstream air-fuel ratio sensor and the actual exhaust air-fuel ratio is calculated based on a correction amount in the sub-feedback control, and a learning control is performed to correct the output value of the upstream air-fuel ratio sensor based on the calculated learned value. Because the learned value is stored in the RAM of the ECU also during stoppage of the engine, for example, even when the output of the upstream air-fuel ratio sensor has not been sufficiently corrected by the sub-feedback control after restarting the internal combustion engine, the output value is appropriately corrected by the learned value. It is thus possible to prevent deterioration in the accuracy of air-fuel ratio control and therefore deterioration of exhaust emissions.
After the execution of a fuel increase or decrease control in which the amount of fuel supplied is increased or decreased irrespective of the target air-fuel ratio during operation of the engine (for example, a fuel cut-off control or fuel increase control at engine start-up), excess oxygen or excess fuel may accumulate in the exhaust purification catalyst. In this case, for example, there is a large difference between the air-fuel ratio of exhaust gas discharged from the combustion chamber and the air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst. Executing the above-mentioned main feedback control, sub-feedback control, learning control, or the like in this state makes it impossible to control the air-fuel ratio in an appropriate manner.
Accordingly, it has been proposed to prohibit learning control for a fixed period of time after completion of the fuel cut-off control (see Japanese Patent Application Publication No. 2005-105834 (JP-A-2005-105834)). This prevents the learned value from being updated when there is a large difference between the air-fuel ratio of exhaust gas discharged from the combustion chamber and the air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst, that is, when the output of the downstream air-fuel ratio sensor is inappropriate. As a result, inappropriate control of the air-fuel ratio is restrained.
As described above, in the sub-feedback control, proportional-integral-derivative (PID) control or proportional-integral (PI) control is performed in order to correct the output value of the upstream air-fuel ratio sensor (and hence the fuel supply amount) based on the output of the downstream air-fuel ratio sensor so that the output value of the upstream air-fuel ratio sensor matches the actual exhaust air-fuel ratio. In the above-mentioned learning control, the learned value is changed based on the value of the integral term used in the integral control in the sub-feedback control. Generally, the larger the value of the integral term, the larger the amount of change in learned value.
On the other hand, as described above, the air-fuel ratio of exhaust gas detected by the downstream air-fuel ratio sensor over a fixed period after the end of fuel cut-off control differs from the air-fuel ratio of exhaust gas discharged from the combustion chamber. In this regard, in the device described in JP-A-2005-105834, although the learning control is prohibited for a fixed period after a fuel cut-off control ends, integral control in the sub-feedback control is not prohibited. Thus; as for the value of the integral term in the sub-feedback control, integration is performed within the fixed period of time based on an air-fuel ratio that deviates from the air-fuel ratio of exhaust gas discharged from the combustion chamber. Therefore, the error in the value of the integral term becomes extremely large by the time this fixed period ends. This means that upon resuming learning control after the end of the fixed period, a learned value is calculated based on the value of the integral term with an extremely large error, making the resulting learned value inappropriate. As a result, exhaust emissions deteriorate.